1. Field of the Invention
The present invention relates to a display apparatus such as a projector and a television monitor.
2. Description of the Related Art
When displaying analog video signals for documents and graphics produced by a computer on a display apparatus, it is important to match a quantization clock and an effective image area, which are needed to quantize the analog video signal in the computer, with those in the display apparatus. In order to perform proper quantization, it is necessary to optimize a frequency of the quantization clock (hereinafter referred to as a “quantization frequency”) and a phase of the quantization clock (hereinafter referred to as a “quantization phase”), and thus a function of automatically performing such optimization, which is a so-called automatic quantization clock adjustment function, has been proposed.
Japanese Patent Laid-Open No. 11-177847 discloses an apparatus having such an automatic quantization clock adjustment function. This apparatus performs for each phase a process to acquire an accumulated value of differences between video signal values of one or more pairs of pixels adjacent to each other in one frame of an input video signal. As shown in FIG. 6, since proper quantization increases the accumulated value of the video signal value differences and improper quantization decreases the accumulated value, the apparatus adjusts the quantization frequency and quantization phase such that the acquired accumulated value of the video signal value differences becomes maximum.
Moreover, Japanese Patent Laid-Open No. 2000-020008 discloses an apparatus having an automatic quantization clock adjustment function that accumulates, in order to acquire the accumulated value described in Japanese Patent Laid-Open No. 11-177847, square values of the video signal value differences.
The apparatus disclosed in Japanese Patent Laid-Open No. 11-177847 is based on a premise that, as shown in FIG. 6, the input video signal includes some parts where an inclination of video signal level change reverses at each pixel, and improves adjustment accuracy of the automatic quantization clock adjustment function as such parts increases. However, for an input video signal including no or few parts where the inclination of the video signal level change reverses at each pixel, the accumulated value of the video signal value differences in the proper quantization and that in the improper quantization have no difference, as shown in FIG. 7, which makes it impossible to perform sufficient adjustment.
Moreover, the apparatus disclosed in Japanese Patent Laid-Open No. 2000-020008 includes a large scale circuit for calculation of the square value, which increases cost.